Yes, portfolio pages are using the template-portfolio.php. If you choose Template > Portfolio it will override the settings on your theme options and will show all of your portfolio items with the default settings 3 columns.

The portfolio slug option allows you to customize the permalinks for portfolio items a bit. Eg many users don’t want to call their portfolio entries “portfolio-items” but the (standard) slug suggests this name. Eg you could insert “projects” instead and then the permalink will be my-website.com/projects/your-single-entry-name instead of my-website.com/portfolio-items/your-single-entry-name